Scanning antenna array wherein feed utilizes dispersive elements to provide nonlinear scan-frequency relationship



K. MILNE 3,324,475 SCANNING ANTENNA ARRAY WHEREIN FEED UTILIZESDISPERSIVE ELEMENTS June 6, 1967 TO PROVIDE NONLINEAR SCAN-FREQUENCYRELATIONSHIP Filed Feb. 13, 1964 3 Sheets-Sheet 1 TRMIIHIUI HIP June 6,1967 K. NE

SCANNING ANTENNA A Y WHEREIN 1 D UTILIZES DISP TO PROVID ONLINEAR CAI-FREQUENCY REIIA Filed Feb. 15, 1964 K. MILNE 3,3

EHSIVE ELEMENT June 6, 1967 SCANNING ANTENNA ARRAY WHEREIIN FEEDUTILIZES DISP 5 TO PROVIDE NONLINEAR SCAN-FREQUENCY RELATIONSHIP FiledFeb. 13, 1964 3 Sheets-Sheet 5 REF/VH7 rm'mzri mum United States Patent3324 47s SCANNING ANTENNA ARRAY WHEREIN FEED urruzas DISPERSIVE ELEMENTST0 PRO- This invention relates to directional aerial systems and moreparticularly to aerial systems giving a directional beam, the directionof which depends on the frequency of the signals fed to or received bythe aerial system.

According to this invention, a directional aerial system comprises anarray of radiating (or receiving elements) fed by (or feeding) adispersive transmission line or a line including dispersive elements. Adispersive transmission line or dispersive element is a transmissionline or element having group delay which is dependent on the frequencyof the applied signal.

It has previously been proposed to use a serpentine waveguide withradiating slots as a directional aerial. Such a serpentine waveguide, byproviding, for example, radiating slots or directional couplers feedingradiating apertures at corresponding points in a repetitive series ofS-shaped bends in the line, gives a length of transmission path betweensuccessive radiators very much greater than the direct physical distancebetween the radiators; thus the relative phase of the radiation from thevarious radiators varies greatly with frequency and so the direction ofthe resultant beam depends on the frequency. To a first order, however,the beam angle is a linear function of frequency. In the arrangement ofthe present invention, however, the transmission line itself isdispersive or contains dispersive elements and thus, by suitable choiceof line or of dispersive elements in the line, it is possible to obtaina non-linear relationship between frequency and beam angle.

This form of aerial system finds particular application in microwaveradar apparatus where it is often required to effect repetitive scanningof a beam over a limited angular extent. The aerial system in this casewould commonly be used both for radiation and reception by employing asuitable duplexer. As an example, an aerial system as described aboveand consisting of an array of radiating (or receiving) elements arrangedin a vertical line might be used to produce a beam which is sharplydirectional in the vertical plane and which can be scanned in a verticalplane by changing the frequency. Such a vertical array may be combinedwith a reflector, e.g. a parabolic cylinder, to make the beam sharplydirectional in the horizontal plane and the whole aerial system may thenbe rotated to give a beam which is scanned rapidly up and down inelevation and is more slowly scanned in azimuth. In such radarapparatus, a short duration pulse may be employed, successive pulsesbeing on different frequencies to efi'ect the vertical scanning, or thepulses may be of sufficiently short duration to contain all thefrequency components corresponding to the desired angular coverage.Alternatively, a long duration frequency modulated pulse may beemployed, for example, as in the radar system described in thespecification of co-pending application Ser. No. 344,781, now Patent No.3,266,038.

Instead of using a parabolic reflector, a number of sets of radiatingelements may be used, the elements in each set being fed by a dispersivetransmission line or a line including dispersive elements and thevarious sets being arranged side by stide. The various sets of radiatingelements may be fed from a common transmission line via. controllablephase shifters; for example electri- 3,324,475 Patented June 6, 1967cally controlled phase shifters may be employed. If the lines arevertical change of frequency produces an elevation scan whilst change ofphase produces an azimuth scan.

In a simple form, the aerial system of the present invention maycomprise a sraight or serpentine form waveguide with dispersive elementsbetween successive radiating slots spaced along the Waveguide. It isoften convenient to describe the form of a complex microwave impedancein a waveguide by reference to the analogous lumped circuit elements ina two-wire transmission line and each dispersive element in thewaveguide may be considered as the microwave analogue of a latticefilter for a two-wire line having an inductance and capacitance in shuntin each of the two lines and having an inductance and capacitance inseries between the input end of each wire and the output end of theother wire. Such a lattice filter network gives a uniform amplitude ofresponse over a wide range of frequencies but the phase variesnonlinearly with frequency and hence the time delay varies withfrequency. The network remains substantiauy matched however as thefrequency varies.

The dispersive elements may be adjustable so that the relationshipbetween frequency and beam angle may be varied; for example the elementsmay be electrically eon. trollable.

In the following description reference will be made to the accompanyingdrawings in which:

FIGURE 1 is a diagram illustrating a dispersive aerial system;

FIGURE 2 is an equivalent lumped circuit diagram for explaining theoperation of a dispersive element;

FIGURE 3 is a perspective view, partly cut away, showing part of anaerial system;

FIGURE 4 is a perspective view illustrating part of another form ofaerial system;

FIGURE 5 is a diagram illustrating a dispersive linear array arrangedfor scanning in a vertical plane in accord ance with the frequency of aninput signal and mechanically scanned in a horizontal plane; and

FIGURE 6 is a diagram used for explanatory purposes.

FIGURE 1 illustrates diagrammatically a number of radiating elements 10which typically are arranged in a Vertical plane one above another andwhich are fed from a transmitter 11 via an input waveguide 12 ofgenerally serpentine form. Commonly this type of aerial would be usedfor both transmitting and receiving, a suitable duplexer 13 beingprovided so that received signals from the waveguide 12 are fed from theduplexer 13 to a receiver 14. For convenience in terminology, however,in the following description reference will be made more particularly tothe transmitting conditions in which signals are fed from thetransmitter 11 through the waveguide 12 to the radiating elements 10.The various radiating elements are coupled to the waveguide 12 bydirectional couplers 15 and short lengths of waveguide 16. Thedirectional couplers 15 have coupling values chosen to suit the desiredaperture illumination, enabling the proportion of the transmitted energyfed to each radiating element to be determined in accordance with therequired conditions. As shown in FIGURE 1, each radiating element 10 isat one end of the short waveguide 16 and the other end of this waveguideis terminated by a matched load 17, this waveguide 16 being coupled bythe directional coupler 15 to the main waveguide 12. The end of the mainwaveguide 12 remote from the transmitter 11 is terminated in a matchedload 18. The main waveguide 12 is of serpentine form and has adispersive element 19 between each of the couplers 15. The mechanicalconstruction of these dispersive elements will be described later withreference to FIGURE 3 and for the present it may be stated that such adispersive element in its simplest form may comprise a 3 db directionalcoupler with two of its de-coupled arms constituting signal input andoutput arms and with the other two arms terminated in identical resonantcavities (each shown as a filter 21 and short circuit 22).

FIGURE 2 shows the equivalent lumped circuit elements in a two-wiretransmission line; each dispersive element 19 may be considered as themicrowave analogue of a lattice filter for a two-wire line having aninductance and a capacitance 31 in shunt in each of the two lines 32 and33 and having an inductance 34 and capacitance 35 in series between theinput end of each line and the output end of the other line. This filtercontains resonant and anti-resonant circuits. Assuming that themagnitudes of each of inductances 30 is L /2, each of the capaeitances31 is 2C each of the inductances 34 is 2L and each of the capacitances35 is Cg/Z, then in the simplest case the tuned circuits are made tohave the same resonant frequency f Thus I 1 f0 l l Z W 2 2 The sharpnessof resonance factor m is given by if C2 The phase shift B at a frequencyf is given by Such a lattice filter network gives a uniform amplitude ofresponse over a wide range of frequencies but the phase variesnon-linearly with frequency and hence the timedelay varies withfrequency. The form of the relationship between the differential delayand frequency can be varied by choice of the two parameters available inthis filter. The network remains substantially matched however as thefrequency varies. In a microwave analogue using a 3 db directionalcoupler with two arms terminated in resonant cavities, the phasecharacteristic is determined by the cavity and it will be appreciatedthat, by the use of complex forms of cavities, it is possible to controlthe dispersive characteristics. Alternatively or additionally thetransmission line may be reactively loaded to provide a dispersivecharacteristic.

Referring now to FIGURE 3 the various radiating elements are illustratedas radiating apertures 40. In FIGURE 3 five such apertures are shown butit will be appreciated that typically a very much greater number mightbe employed, for example, 100. Each of the apertures 40 is coupled by atransition section 41 to one end of short rectangular waveguide 42 theother end of which is terminated in a matched load 43. The waveguides 42are coupled by coupling slots 44 to spaced points along the length of amain feed guide 45 of serpentine form. This waveguide 45 corresponds tothe waveguide 12 of FIGURE 1 and the coupling slots 42 for the variousradiating apertures 40 coupled into portions of the waveguide 45separated by the necessary dispersive elements. The input end of thewaveguide 45 is illustrated at 46. The other end 47 is terminated in amatched load. The waveguide 45 is arranged like the waveguide 12 inFIGURE 1 with 3 db couplers 48 and filter elements, consisting of posts49, with short circuits in the portions of the waveguides beyond theposts that is to say at the left-hand of FIGURE 3.

The arrangement of FIGURE 3 employs a rectangular waveguide and isarranged for radiating (or receiving) linearly polarised signals whichare transmitted through the waveguide in a TE mode. It is often arequirement however in radar systems to be able to radiate signalspolarised in two orthogonal planes; depending on the relative phase ofthe two signals, the resultant radiation may be circularly orelliptically or linearly polarised. FIGURE 4 illustrates a constructionof a dispersive aerial for radiating signals in two orthogonal planes ofpolarisation simultaneously. Referring to FIGURE 4 there are shown twoserpentine feed guide systems 50, 51 each generally similar to theserpentine feed guide 45 of FIGURE 3 and incorporating dispersiveelements such as are shown in FIGURE 3. These two serpentine feed guides50, 51 are arranged side by side and each radiating aperture is fed fromboth these feed guides. FIGURE 4 illustrates one such radiating aperture52 in the form of a fin-loaded square-section horn fed via apolarisation coupler 53 from two square-section waveguides 54 and 55.The square waveguide 54 takes its input from the serpentine feed guide50 by means of a combined coupler and differential phase equaliser 56and having an output waveguide of rectangular section (corresponding tothe waveguide 42 of FIGURE 3) which leads via a twist 57 to arectangular-to-square waveguide transition 58 and an E-plane singlemitre bend 59 to the aforementioned square-section waveguide 54. Thesquare-section waveguide 53 is fed from the serpentine feed guide 51 bya coupler 60 feeding the signals into a rectangular waveguide whichleads via a rectangular-to-square transition 61 into the squaresectionwaveguide 55. The relative amplitude and phase of the signals into thetwo feeds 50 and 51 is adjusted to provide the desired polarisation atthe radiating apertures; this may be done using known techniques foradjustable polarisation radar systems.

These aerial systems of FIGURES 3 and 4 consisting of a stack ofradiating elements with the dispersive feeds can give a directional beamwhich is sharply beamed in one plane but the direction of which, as willbe further explained below, is dependent on the frequency of the signal.Assuming that the radiating elements are stacked in a vertical line, thebeam will be sharply directional in a vertical plane. Commonly it isrequired that a radiated beam should be directional in two planes andFIGURE 5 illustrates diagrammatically a typical way in which the stackedradiating elements of FIGURES 3 or 4 might be used in a radar system.Referring to FIGURE 5 the radar system is illustrated diagrammaticallyas comprising a transmitter 70 and a receiver 71 coupled via a duplexer72 to a serpentine feed 73 which may be of the form shown in FIGURES 3or 4 and which has a series of radiating elements 74. The radiation fromthese elements is directed into a cylindrical parabolic type reflector75 for forming the horizontal beam and the complete aerial system isrotated about a vertical axis as indicated diagrammatically at 76. Itwill be seen that the beam is directional in the horizontal plane in amanner determined by the reilector 75 and is rotated mechanically forscanning in this plane. In the vertical plane the beam shape isdetermined by the feed system 73, 74, but the elvation angle will dependon the frequency. Such an aerial might be used with radar apparatus inwhich successive pulses are radiated on different frequencies to effectthe vertical scanning or the pulses may be of sufliciently shortduration to contain all the frequency components corresponding to thedesired angular coverage. Alternatively long durationfrequency-modulated pulses may be employed as described, for example inthe specification of co-pending application Ser. No. 344,781, now PatentNo. 3,266,038.

FIGURE 6 is an explanatory diagram used in the following analysis of therelationship between beam angle and group delay and of the radiationpattern of a dispersive aerial system such as has been previouslydescribed. Re-

ferring to FIGURE 6 there is shown diagrammatically a linear arraycomprising a serpentine feed 80 with radiating elements 81. Thedirection of the main beam is indicated diagrammatically by the dashlines 82 which are at an angle to the normal line of the array. The beamangle 6 is determined from the equation d0 t n a a]? sec w r a where -1dtbtf) (f)-y T is the group delay of the feeder at frequency J. Foroperation close to broadside (0:0), Equation 2 reduces to:

d0 X Zimand since A/d is the approximate value of beamwidth we obtainthe important relation: Beamwidths per c./ s. frequency change' GroupDelay (5) For normal waveguide feeds, the group delay is m if E x (awhere s=total feeder length c=fk=velocity of electro magnetic waves infree space A :guide wavelength.

Waveguide group delay thus reduces as the frequency increaes, but thedispersion is very small except near the cut-off frequency.

The radiation pattern received at a point at a considerable distance Rfrom the centre of the array shown in FIGURE 6 is proportional to wherea is the excitation of the mth radiator, and the phase is measured withrespect to the input feeding point.

For symmetrical amplitude distributions (i.e. a =a it is convenient towrite (7) in the form:

where n e f Ea... Pj{ Sm m=o n A 2 is a real function which defines theshape of the radiation pattern.

If i and A are the frequency and corresponding wavelength at which thebeam is normal to the array (i.e. fif is an even multiple of Zmr), thepattern at becomes where g(sin 0, f0) =22 m EXP The pattern at any otherfrequency f can then be written in terms of the pattern at I as:

E(sin 0,

where The beamwidth at frequency f is thus f /f times the beamwidth at Iand the beam maximum occurs at slnaf d which is the same result as thatgiven in Equation 2. Differentiation of the phase term in Equation 12shows that the total group delay at the target is simply R 1 FW 15)Equation 12 gives the variation of field strength with angle at a fixedfrequency, or the spectrum obtained at a fixed angle when the array isfed with a uniform spectrum. If the array is uniformly illuminated(spatially) for example, Equation 10 reduces to and the pattern atfrequency f is where (10) is given by Equation 13. The angular width ofa major lobe between zeroes at a fixed frequency is 20 where 9 lm i i En+1dfn+1d 18) whilst the width of the major lobe in the spectrum at afixed angle is 2B c.p.s. where B is given by the equation a .lh-B f n+1(19) in which f is the frequency giving maximum response at the angle ofinterest. If D(j) does not change too rapidly over the interval 2B,Equation 19 yields &7! n 1 D( f (20) The half-power beamwidth is thusapproximately Vd and the half-power bandwidth is approximately equal tothe reciprocal of the aerial group delay.

I claim:

1. A directional aerial system comprising an array of radiating elementsfed by a transmission line including dispersive elements so that becauseof the dispersion the sweep of the radiation beam of said array isnon-linearly related to the frequency.

2. A directional aerial system comprising a waveguide with a series ofradiating elements coupled to the guide at spaced points along itslength and having dispersive elements spaced along the waveguide betweenthe successive couplings to the radiating elements so that because ofthe dispersion the sweep of the radiation beam of said system isnon-linearly related to the frequency.

3. A directional aerial system as claimed in claim 2 wherein thewaveguide is of serpentine form.

4. A directional aerial system as claimed in claim 2 wherein eachdispersive element comprises a 3 db directional coupler with a pair ofde-coupled arms connected to resonant cavities and the other two armsconstituting signal input and output arms so that the signaltransmission path through said waveguide passes into one of said otherarms and out through the other.

5. A directional aerial system comprising a main waveguide, a series ofauxiliary guides coupled by directional couplers to said main guide atspaced points along its length, dispersive elements spaced along thewaveguide between the successive couplings to the auxiliary guides sothat because of the dispersion the sweep of the radiation beam of saidsystem is non-linearly related to the frequency, said auxiliary guideseach having a radiating aperture.

6. A directional aerial system as claimed in claim 5 wherein thewaveguide is of serpentine form.

7. A directional aerial system comprising a number of radiating elementscoupled to spaced points on a signal transmission path for feeding allsaid elements, which signal transmission path between each of saidspaced points includes a waveguide from one of the points leading to afirst arm of a 3 db directional coupler and a waveguide leading from asecond arm of the coupler tie-coupled from the first arm, the other twoarms of the coupler each containing a filter and being terminated in ashort circuit, some of said elements including a dispersion element sothat the sweep of the radiation beam of said system is non-linearlyrelated to the frequency.

8. A directional aerial system comprising a number of radiatingelements, coupling means for feeding to each radiating element fromseparate input signals with different planes of polarisation, a pair oftransmission lines each coupled at spaced points along its length to oneof the inputs of the successive radiating elements, dispersive elementson said lines situated between each adjacent pair of coupling points sothat because of the dispersion the sweep of the radiation beam of saidsystem is non-linearly related to the frequency and means for feedingsignals of the same frequency in a predetermined amplitude and phaserelationship to the two transmission lines.

9. A directional aerial system comprising a signal transmission path, anumber of radiating elements arranged in line and coupled to spacedpoints on said signal transmission path for feeding said elements, whichsignal transmission path between each of said spaced points includessignal delay means having a non-linear relationship between delay timeand frequency so that because of the said non-linear relationship thesweep of the radiation beam of the said radiating elements isnonlinearly related to frequency.

10. A directional aerial system as claimed in claim 9 wherein saidsignal delay means comprise a 3 db directional coupler with twode-coupled arms of the coupler each containing a filter and beingterminated in a short circuit.

11. A directional aerial system as claimed in claim 9 wherein saidradiating elements are arranged in an upright line to give beaming in avertical plane and feed a reflector shaped to give beam in a horizontalplane.

12. A directional aerial system as claimed in claim 11 and mounted forrotation to scan the beam about a vertical axis.

13. A directional aerial comprising a number of sets of radiatingelements, the elements in each set being fed by a dispersivetransmission line and the various sets being arranged side by side, andbecause of the dispersion the sweep of the radiation beam of said aerialis nonlinearly related to the frequency.

14. A directional aerial as claimed in claim 13 wherein the various setsof radiating elements are fed from a common transmission line viacontrollable phase shifters.

15. A directional aerial system comprising a plurality of radiatingelements evenly spaced apart and fed from evenly spaced points on aserpentine feed system to give a directional beam, which feed system,between suocessive spaced points, has dispersive elements whereby thereis a non-linear relationship between the direction of the radiated beamand the frequency of signals applied to said feed system.

16. A directional aerial comprising a number of sets of radiatingelements, the elements in each set being fed by a transmission lineincluding dispersive elements and the various sets being arranged sideby side and because of the dispersion the sweep of the radiation beam ofthe said aerial is non-linearly related to frequency.

17. A directional aerial as claimed in claim 16 wherein the various setsof radiating elements are fed from a common transmission line viacontrollable phase shifters.

References Cited UNITED STATES PATENTS 2,530,580 11/1950 Lindenblad343777 X 2,605,413 7/1952 Alvarez 343-854 X 2,878,472 3/1959 Stems343-853 3,020,549 2/1962- Kales et al. 343-77l 3,041,605 6/1962 Goodwinet al. 343 3,105,968 10/1963 Bodmer 343-77l 3,142,028 7/1964- Wanselow33383 X HERMAN KARL SAALBACH, Primary Examiner.

M. NUSSBAUM, Examiner.

R. F. HUNT, Assistant Examiner.

1. A DIRECTIONAL AERIAL SYSTEM COMPRISING AN ARRAY OF RADIATING ELEMENTSFED BY A TRANSMISSION LINE INCLUDING DISPERSIVE ELEMENTS SO THAT BECAUSEOF THE DISPERSION THE SWEEP OF THE RADIATION BEAM OF SAID ARRAY ISNON-LINEARLY RELATED TO THE FREQUENCY.